The present invention relates to a hydraulic circuit for recovering energy, which circuit comprises at least one hydraulic motor, two main ducts for feeding or discharging said at least one motor, a low-pressure fluid source, and a high-pressure accumulator forming a high-pressure fluid source, the circuit being suitable for operating in energy recovery mode in which the feed main duct is connected to the low-pressure fluid source and the discharge main duct is connected to the high-pressure accumulator, and in energy delivery mode in which the feed main duct is connected to the high-pressure accumulator and the discharge main duct is connected to the low-pressure fluid source.
It is known that such circuits can be fitted to vehicles for the purpose of limiting energy consumption. Such vehicles are “hybrid” vehicles, each such vehicle having a conventional propulsion engine (e.g. of the internal combustion engine type) and a circuit of the above-mentioned type in which the hydraulic motor(s) can be coupled mechanically to the propulsion device, e.g. by being coupled to the drive outlet of said device, or indeed to a wheel of the vehicle. At cruising speed, the vehicle is normally driven by its conventional propulsion engine. Energy recovery takes place during braking, during which the hydraulic motor is driven by the propulsion of the vehicle that is being braked, so that said hydraulic motor operates as a pump and feeds the high-pressure accumulator with fluid. Energy delivery takes place in particular during an acceleration phase, during which the vehicle is accelerating and during which the hydraulic motor is activated once again and is fed with high-pressure fluid coming from the accumulator, so as to deliver drive torque for assisting propulsion of the vehicle.
Such “hybrid” systems are known to be advantageous in reducing fuel consumption.
Conventionally, a low-pressure accumulator is used as the low-pressure fluid source. Unfortunately, the shape and the size of such a low-pressure accumulator can pose compactness problems.
It is possible to choose a booster pump as the low-pressure fluid source, the feed of the booster pump then being connected to a reservoir at atmospheric pressure, which reservoir is relatively little constrained as to its shape and is therefore easier to incorporate into the vehicle. The booster pump is dimensioned to perform optimum boosting of the cylinders of the hydraulic motor(s) during energy recovery, while the flow rate is at its maximum. Such pumps are conventionally of fixed rating, i.e. they are associated with a pressure limiter having fixed setpoint, limiting the delivered pressure to a single predetermined value.
The hydrostatic braking torque achieved by the hydraulic motor(s) during an energy recovery phase and the drive torque delivered by said motor(s) during an energy delivery phase is equal to the product of the cubic capacity of the motor multiplied by the difference between the high pressure of the high-pressure accumulator and the boosting low pressure of the pump. Said high pressure is determined by the accumulator, as a function of its level of charge, while, as indicated above, said low pressure is set by the fixed rating of the pump. In addition, if the cubic capacity of the motor is fixed, it is not possible to adjust the hydrostatic braking torque or the drive torque to match real needs.
In order to avoid braking that is too sudden during an energy recovery phase, or drive assistance that is too sudden during an energy delivery phase, the hydraulic motor(s) tend(s) to be overdimensioned, and the outlet torque therefrom is therefore relatively limited.
As a result, the possibilities of storing fluid during an energy recovery phase and the possibilities of delivering drive assistance during an energy delivery phase are relatively limited, which naturally adversely affects the efficiency of the circuit and thus the overall performance thereof. In other words, the reduction in fuel consumption is not optimized.
Naturally, when it is a low-pressure accumulator that is used as a low-pressure fluid source, the above-mentioned difficulties are also encountered, because the low pressure is then determined by the low-pressure accumulator, without any possibility of adjustment.
An object of the present invention is to remedy those drawbacks or, at least, to attenuate them significantly.
This object is achieved by the fact that the low-pressure fluid source comprises a high-flow-rate booster pump suitable for delivering, from a pressure-free reservoir, a flow-rate of fluid that is sufficient to feed fluid to said at least one hydraulic motor while said motor is at maximum speed in energy recovery mode, and an adjustable pressure limiter suitable for being controlled so as to cause the pressure of the fluid at the low-pressure fluid source to vary.
It can be understood that, by causing the low-pressure of the fluid of the low-pressure fluid source to vary by means of the adjustable pressure limiter, it is possible for the torque of the motor to be varied even if the cubic capacity of said motor is fixed, it being recalled that said torque is the product of said cubic capacity multiplied by the difference between the high pressure and the low pressure. The fact that the pressure limiter is adjustable means that it is a progressive valve, whose position can be adjusted finely as a function of the pressure needs. Thus, the adjustable pressure limiter makes it possible, regardless of the type of hydraulic motor used, to set the low pressure to a level necessary for adjusting the drive torque and for avoiding cavitation, both in an energy recovery phase and in an energy delivery phase.
For example, during an energy recovery phase, it is possible to choose to set the outlet pressure of the low-pressure fluid source to a level necessary for feeding the hydraulic motor(s) (the speed of which is determined by the conventional propulsion of the vehicle) without cavitation, thereby enabling the motor(s) to pump a maximum quantity of fluid so as to optimize recharging of the high-pressure accumulator. During the energy delivery phase, it is possible to choose to set the low pressure to a minimum level so as to obtain high assistance torque.
Advantageously, the circuit further comprises means for activating or deactivating the high-flow-rate booster pump, which means are preferably associated with a check valve preventing fluid from returning towards said pump.
This makes it possible to consume the energy necessary for driving the pump only when said pump is useful for recovering energy, allowing said pump to be deactivated otherwise.
Advantageously, the circuit further comprises means for detecting at least one indicator parameter that indicates the state of the circuit, and for controlling the adjustable pressure limiter as a function of said at least one parameter. For example, said indicator parameter comprises the level of charge of the high-pressure accumulator.
These features make it possible to adjust the low pressure more finely so as to adjust the braking torque or the assistance drive torque to match the real need, as indicated by the above-mentioned parameter.
For example, in energy recovery mode, the high pressure of the circuit, which pressure is the pressure necessary for filling the high-pressure accumulator, increases with increasing level of charge thereof. Without adjusting the low pressure, the braking torque thus increases with the level of charge of the accumulator. This is not always desirable, in particular because such an increase might occur at the end of a vehicle braking phase, even though the speed of the vehicle has already decreased very significantly, so that constant braking torque would suffice.
By means of the above-mentioned features, it is possible, conversely, to increase the outlet pressure of the low-pressure fluid source at the same time as the pressure of the high-pressure accumulator increases, so as to stabilize the braking torque.
Advantageously, the circuit further comprises means for controlling the adjustable pressure limiter as a function of an operating setpoint for operation of said at least one hydraulic motor. For example, the operating setpoint relates to the outlet torque of said at least one motor.
This makes it possible, in particular, to cause the motor(s) to operate in energy recovery mode so as to recharge the high-pressure accumulator, while the braking torque demand is low, and, symmetrically, to set the assistance drive torque so as to avoid jolting in the propulsion and, and to cause the motor(s) to operate in energy delivery mode, even when torque demand is not very high. It is thus possible to increase the opportunities for using hydraulic assistance, thereby further reducing the fuel consumption of the conventional propulsion.